In recent years, automated, spring-propelled dual stage automated tissue sampling devices have greatly simplified retrieval of tissue specimens for biopsy purposes. An example of such an instrument is disclosed in U.S. Pat. No. 4,699,154 to Lindgren et al. That patent discloses a driving unit having a pair of sequentially driven, spring-propelled slides for driving a coaxial needle assembly. The coaxial needle assembly comprises a cannula having a sharpened forward end and a stylet telescopically received within the cannula. The stylet and cannula each have needle heads mounted to their rearward ends which are removably coupled to the spring-propelled slides of the driving assembly. When the driving unit is actuated, the first slide is released to propel the stylet forward into the target tissue. As the tip of the stylet enters the target tissue, a section of the tissue enters a notch near the forward end of the stylet. At the instant the stylet reaches the forward end of its travel, the second slide is released, propelling the cannula forward over the stylet, shearing off the tissue specimen and capturing it within the forward end of the cannula. The needle is then withdrawn from the patient, and the needle assembly is removed from the driving unit to permit the tissue specimen to be recovered for biopsy.
Despite its significant advantages over prior art biopsy needles, the biopsy instrument disclosed in the aforementioned U.S. Pat. No. 4,699,154 suffers certain disadvantages when used in conjunction with CT scan. After the physician has directed the needle assembly to the desired tissue location identified by the CT scan, and before actuating the driving unit to retrieve a tissue specimen, the patient must be rescanned to verify the location of the needle tip with respect to the lesion. This rescanning cannot be done with the needle coupled to the driving unit, because the weight of the drive mechanism would not be supported by the needle sticking in the patient, and because the needle drive mechanism would not fit within the gantry of the CT scanner. Thus, it is necessary to uncouple the needle assembly from the driving unit to rescan the patient.
A problem arises when the coaxial needle assembly is not coupled to its driving unit. Since the needle assembly is not intended for use separate from the driving unit, there is no mechanism by which the needles can be locked relative to one another. Since the stylet telescopes freely within the cannula, it is very easy for the needles to move relative to one another. Consequently, with the needle assembly uncoupled from the driving unit to permit CT scan, normal movements by the patient, e.g. aspiration, can cause relative displacement between the stylet and cannula. Since the slides of the driving unit are a predetermined distance apart, the needle head must be a corresponding distance apart for the needle assembly to be recoupled to the driving unit after CT scan. This requirement may necessitate the stylet and cannula to be moved relative to one another to conform the spacing between the needle heads to the spacing between the slides of the driving unit. Such movement may cause displacement of the tip of the needle with respect to the target tissue, thereby negating the confirmation obtained by the CT scan and possibly leading to an inaccurate tissue sample.
Consequently, there arose a need for a mechanism whereby the cannula and stylet of a needle assembly can be retained in predetermined, spaced apart relation while the needle assembly is uncoupled from its associated driving unit to permit CT scan.
An article by Parker et al., which appeared in RADIOLOGY, Vol. 17, entitled Image-directed Percutaneous Biopsies with a Biopsy Gun, discloses the use of a spacer member comprising a short length of longitudinally slit tubing designed to fit over that portion of the stylet between the cannula needle head and the stylet needle head. The spacer member maintains the stylet and cannula in the desired relationship when the needle assembly is not mounted to the driving unit. With the spacer member installed and the needle assembly uncoupled from the driving unit, the physician is able to manually direct the needle assembly to the site of the lesion with the spacer maintaining the stylet and cannula fixed relative to one another. With the needle in situ, the patient is rescanned to verify the location of the tip of the needle assembly with respect to the lesion. If necessary, the needle is repositioned, and the patient is rescanned. When the needle is properly positioned and the needle location has been verified, the spacer is removed from the needle assembly, and the cocked drive unit is mounted to the needle assembly. The biopsy instrument is then fired in the conventional manner to retrieve a tissue sample.
This approach suffers certain disadvantages in that, since the needle spacer is removed prior to the needle assembly being coupled to the driving unit, it is possible to experience some relative movement of the stylet and cannula, and hence possibly movement of the needle tip with respect to the target tissue, while attempting to couple the needle assembly to the driving unit. Further, if the needle assembly is coupled to the driving unit before the spacer is removed, it is easy for the physician to forget to remove the spacer before attempting to fire the instrument. Since the spacer maintains the respective needle heads at a fixed separation, the requisite relative telescoping movement of the coaxial needle assembly cannot be achieved with the spacer still mounted. Thus, when the physician actuates the trigger of the driving unit to fire the needles, the spacer may prevent either of the needles from advancing. Alternatively, triggering the driving unit with the spacer still mounted could result in both needles being fired simultaneously, rather than sequentially. In such an event, the tip of the stylet would never telescope forward of the cannula to expose the tissue receiving recess adjacent the forward end of the stylet, and no tissue specimen would be retrieved. To compound the problem, the instrument may appear to the physician to have been fired normally, so the physician will assume a tissue specimen has been retrieved and will withdraw the needle from the patient before realizing that a tissue specimen has not been captured. As a result, the physician would have to repeat the entire procedure, including repositioning the needle and rescanning to verify the position of the needle tip. Such a mistake would increase discomfort to the patient and increase the cost of the procedure, since physician time required to perform the procedure is increased.